1. Field of the Invention
This invention relates to methods of fabrication of a photomask and more particularly to methods of fabrication of an attenuating phase shifting photomask using a modulated electron beam to expose a layer of resist.
2. Description of the Related Art
As photolithography advances to 0.35 microns and below, new technologies are required to provide accurate photolithographic patterns. One such new technology involves the use of attenuating phase shifting materials to form mask images. The attenuating phase shifting material both shifts the phase of light passing through the material, usually by 180.degree., and provides a partial absorption of the light passing through the material. Attenuating phase shifting masks often use opaque material, usually chrome, in conjunction with the attenuating phase shifting material to prevent unwanted exposure of photoresist in border regions or corner regions of masks where multiple exposure steps can result in unwanted exposure of photoresist.
U.S. Pat. No. 5,738,337 to Tzu et al. describes a method of fabricating a attenuating phase shifting photomask with a chrome border using two exposure doses of a single electron beam.
U.S. Pat. No. 5,480,747 to Vasudev describes an attenuated phase shifting mask using buried absorbers.
U.S. Pat. No. 5,503,951 to Flanders et al. describes an attenuating phase shifting mask and fabrication method for a mask having recessed attenuating phase shifting regions.
U.S. Pat. No. 5,565,286 to Lin describes a structure and fabrication method for a phase shifting mask.
A paper by G. Owen and P. Rissman, Journal of Applied Physics, Vol. 54, No. 6, June 1983, pages 3753-3581 describes proximity effect and methods of correcting for proximity effect.